近年京都議定書即將於2012年到期，為儘速達成新的國際排放減量協議，UNFCCC於2007年起便展開後京都時期的氣候協商，重要議題包含2012年後減量目標、調適資金、碳交易以及技術移轉等，而在2009年底哥本哈根會議中，由美國等五國協商出的哥本哈根協定並不具法律規範之性質，而在該協定的擬定過程中，美國一直扮演著相當重要的主導角色，故美國後京都時期的動向普遍受到國際關注。若從美國歷年相關立法來看，其潔淨空氣法（CAA）在Massachusetts v. EPA訴訟案後雖已納入溫室氣體管制，但因溫室氣體的影響層面過於廣泛，並不適合規範作為管制溫室氣體的主要規範，而美國總統歐巴馬在2008年上任後宣布將制訂新的能源與氣候政策作為其任內最優先執行的政策，為回應其綠色新政，眾議院及參議院分別在2009年提出新的氣候法案-美國潔淨能源與安全法（ACES）與潔淨能源工作與美國電力法（CEJAPA），兩法規範即使略有差異，皆不脫離氣候變遷調適措施與潔淨能源科技發展等架構，配合再生能源使用標準（RPS）與再生燃油標準（RFS）的實施，將可協助美國有效達成減量目標。若從UNFCCC與美國之間的互動關係來看，因UNFCCC爭議問題遲遲無法解決，且美國後京都時期之主張容易引起已開發及開發中國家的對立，故美國須藉雙邊或多邊協談方式緩和兩方衝突，以促進UNFCCC協議的達成，此外，後京都時期UNFCCC協議的達成將對美國新氣候法造成影響，因此，美國可在後京都協商中積極推廣其氣候法的優點，並結合自身主張立場來影響國際氣候談判的結果，以利美國未來氣候法的進一步發展。Recently the Kyoto Protocol would be expired at the end of 2012, in order to reach a new agreement of global emission reduction, the post-Kyoto negotiations under UNFCCC have been held since 2007. The major issues include reduction targets after 2012, adaptation fund, carbon trade and technology transfer. The 2009 conference was held in Copenhagen, Denmark. After the conference, delegates approved the Copenhagen Accord, which was discussed by U.S. and other four developing countries, is not legally binding. Even so, the America played an important role in the negoitation process, and most countries around the world usually pay attention to U.S. movements during the post-Kyoto negotiation period.

For the past years, the U.S. Clean Air Act（CAA）regulate greenhouse gas after Massachusetts v. EPA, however, the CAA is not appropriate to regulate greenhouse gas because the influence of greenhouse gas emission is extensive. In 2008, the U.S. President Obama announced that he would set a new energy and climate policy as a top priority during his presidential terms. To respond to Obama’s green policy announcement, the House of Representitives and Senate set two climate laws, American Clean Energy and Security Act（ACES）and Clean Energy Jobs and American Power Act（CEJAPA）in 2009. Although these two laws have many different regulations, they both have a framework about climate mitigation, adaptation and clean energy technology development. If the U.S. government operate the framework with renewable portfolio standards（RPS）and renewable fuel standards（RFS）, they would reach their redution target effectively.

Focusing on the interation between UNFCCC and U.S., the dispute of UNFCCC could not be settled and the U.S. position on climate change would lead to some conflict between developed and developing countries. To reach an agreement among both side, U.S. must use bilateral or multilateral negotiation ways to mitigate the conflict between developed and developing countries. Besides, the UNFCCC agreement in post-Kyoto period may have some influnces on U.S. new climate law. If the U.S. can promote the advantages of their new climate law and stand their climate position to effect the result of the post-Kyoto negotiation, the U.S. new climate laws may be carry out more smoothly.